In electrostatography an image comprising an electrostatic field pattern, usually of non-uniform strength, (also referred to as an electrostatic latent image) is formed on an insulative surface of an electrostatographic element by any of various methods. For example, the electrostatic latent image may be formed electrophotographically (i.e., by imagewise photo-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on a surface of an electrophotographic element comprising a photoconductive layer and an electrically conductive substrate), or it may be formed by dielectric recording (i.e., by direct electrical formation of an electrostatic field pattern on a surface of a dielectric material). Typically, the electrostatic latent image is then developed into an electrostatographic toner image by contacting the latent image with an electrostatographic developer containing an electrostatographic toner. If desired, the latent image can be transferred to another surface before such development. Although such techniques are typically used for black and white reproductions such as copying business correspondence, they are capable of forming a variety of single color or multicolor toner images.
A typical method of making a multicolor copy involves trichromatic color synthesis by subtractive color formation. In such synthesis successive latent electrostatic images are formed on a substrate, each representing a different color, and each image is developed with a toner of a different color and is transferred to a support (receiver). Typically, but not necessarily, the images will correspond to each of the three primary subtractive colors (cyan, magenta and yellow), and black as a fourth color, if desired. For example, light reflected from a color photograph to be copied can be passed through a filter before impinging on a charged photoconductive layer so that the latent electrostatic image on the photoconductive layer corresponds to the presence of yellow in the photograph. That latent image can be developed with a yellow toner and the developed image can be transferred to a support. Light reflected from the photograph can then be passed through another filter to form a latent electrostatic image on the photoconductive layer which corresponds to the presence of magenta in the photograph, and that latent image can then be developed with a magenta toner and transferred to the same support. The process can be repeated for cyan (and black, if desired).
In the systems described previously herein, the toner images may be provided on a support such as paper, film, plastic or glass to which they are permanently fixed. A common technique for fixing such toner images to a support involves employing thermoplastic polymeric toner particles which include a colorant and fusing the particles to the support by the application of heat and pressure thereto. A suitable method involves passing the support with the toner particles thereon through a pair of opposed rolls, one a heated fuser roll and the other a non-heated or heated backup roll.
It is known to use the electrostatographic processes described, to provide transparencies that are primarily intended for viewing by transmitted light, for example, observing a projected image from an overhead projector. In a typical application the viewable fused toner image is either a single color or multicolor image but such viewable image may also have a single color portion and a multicolor portion. An acceptable transparency requires that the colored toner image exhibit good color clarity or chroma. Color clarity or chroma are terms used to describe the quality of an image projected by a transparency and high color clarity or high chroma refers to a faithful reproduction of the original colored image while low color clarity or low chroma refers to less than faithful or inaccurate reproduction of the original colored image. Low color clarity or chroma can result from light scattering or multiple reflections within a colored toner image which in turn results in a color shift upon projection of the color transparency and a failure to faithfully reproduce the colors of the original image. For example, bright yellow in an original image may appear as a muddy yellow.
It is also known that a transparency comprising a colored toner image exhibiting acceptable color clarity can be prepared in a fusing method where the image is pressure contacted with a highly polished heated roll. Such methods also provide toner images that have very high gloss since the toner image surface is smooth and highly reflecting. Unfortunately, toner images having such high gloss are not always desirable. For example, in a reflection color copy comprising a colored toner image it is often desirable to provide a toner image surface that has a low-level luster or fine matte appearance which has been found to be pleasing to a viewer. Such a surface has been achieved by contact fusing colored toner images using fusing surfaces that are textured, for example, textured fusing rolls. Unfortunately, the resultant textured toner image surfaces are known to deleteriously affect color clarity in a color transparency because they exhibit the harmful light scattering or multiple reflections described previously herein.
U.S. Pat. No. 4,791,447 issued Dec. 13, 1988, addresses the problem of providing glossy opaque toner images and high chroma transparencies using a fusing system comprising three roll members which cooperate to form a pair of roll nips.
In light of the previous discussion, it is obvious that it would be desirable to have a fusing method that is sufficiently flexible to provide transparencies comprising color toners having high color clarity and also be capable of providing toner images that exhibit a pleasing low-level luster of the type desired in certain reflection copy prints such as continuous tone reflection copy prints. This invention provides such a fusing method.